As will be familiar to those skilled in the art, in a typical ion implanter a relatively small cross-section beam of dopant ions is scanned relative to a silicon wafer. Traditionally, a batch of wafers was mechanically scanned in two directions relative to a fixed direction ion beam.
With the advent of larger wafers, up to 300 mm in diameter, processing of a single wafer at a time becomes advantageous in terms of cost, reduced wastage etc. Accordingly, it is now desirable to scan an ion beam relative to a silicon wafer by mechanically scanning the wafer in a first direction and electrostatically or electromagnetically scanning or fanning the ion beam in a second direction.
There are a number of different configurations of single wafer processing machines. One example is described in WO99/13488 and other configurations are described in U.S. Pat. Nos. 5,003,183 and 5,229,615. In WO99/13488, the wafer is mounted upon a substrate holder in a process chamber of an implantation device. Attached to, or integral with, the substrate holder is an arm which extends through an aperture in the wall of the vacuum chamber. Mechanical scanning is effected by a scanning mechanism located outside the process chamber. The scanning mechanism is connected with the arm of the substrate holder and allows movement of the arm and hence the substrate holder relative to the process chamber.
To facilitate movement of the moving parts of the scanning mechanism, one or more gas bearings are provided. For example, the end of the arm distal from the substrate support may be attached to a first bearing member which moves reciprocally relative to a second bearing member. This allows the wafer to be mechanically scanned in a plane orthogonal to the ion beam of the ion implanter. Movement of the first bearing member relative to the second bearing member is facilitated via a first gas bearing.
Likewise, the second bearing member may itself be rotatable relative to the process chamber to allow tilting of the substrate support relative to the direction of the ion beam. The second bearing member rotates against a stator mounted upon a flange adjacent the aperture in the wall of the process chamber; a second gas bearing is employed between the stator and the surface of the second bearing member to facilitate this rotation.
For successful operation of the gas bearings, the bearing surfaces must each be flat. Variations in flatness of more than 10 .mu.m or so can cause one of the bearing surfaces to touch the other bearing surface. Whilst the bearing surface of the second bearing member and that of the stator may be made flat to this accuracy, in use the second bearing member is subject to a vacuum on its inner side and to the force of atmospheric pressure on its outer non-bearing surface. This can lead to distortion of the second bearing member, particularly to "dishing" whereby the centre of the second bearing member tends to bow inwardly relative to its periphery. When this happens, the bearing surface of the second bearing member is no longer parallel to the bearing surface of the stator and thus a larger clearance must be maintained between the two surfaces for the gas bearing to operate successfully.
It is an object of the present invention to address this problem. More generally, it is an object of the invention to reduce the problems associated with distortion of the bearing surfaces in a fluid bearing.